III. Gratings
A grating is an optically flat polished surface that has dense parallel grooves. Two
types of gratings are usually encountered, transmission and reflection gratings.
Transmission gratings are seldom used in spectroscopic instruments and almost all
gratings which are used in conventional spectroscopic instruments are of the
reflection type. The groove density can be as low as 80 to several thousand (6000)
lines/mm. Two common types of reflection gratings can be identified:
1. Echellette Gratings: Typical echellette gratings contain from 300 to 2000
lines/mm but an average line density of about 1200 to 1400 lines/mm are most
common. The echellette grating uses the long face for dispersion of radiation. It is the
grating of choice for molecular spectroscopic instruments. As will be shortly
explained and in contrast to prisms, gratings usually have linear dispersion of
radiation.
2. Echelle Gratings: These have relatively coarse grooves (~80-300 lines/mm). They
use the short face for dispersion of radiation and are characterized of very high
dispersion ability.
Gratings can be made concave which have the advantage of excluding collimation
mirrors, resulting in simpler instrumental designs. An important category of gratings
is the one manufactured by laser technology. The thus made gratings are called
holographic gratings and are characterized by a high degree of perfection and very
high line density (up to 6000 lines/mm), in addition to larger size. They have become
a low cost product as well.
Gratings, other than holographic, are made by mechanical ruling of hard surfaces
using a mechanical device like a diamond tool. The first grating to be made takes too
much time and effort and is called a master grating. Replica gratings are made from
the master by a liquid resin casting process. The liquid resin takes the exact shape of
the grooves and coated with a reflective metal like aluminum, gold or platinum.
Replica gratings are easy to make and thus after the master is made, the process of
production is extremely simple and fast.
Dispersion by Gratings
We can visualize what is going on when radiation hits the surface of a grating. Our
discussion will be focused on echellete grating but conclusions are fully applicable to
all reflection gratings as well.
1'
2
2
1
X
r
C
D
i
A
B
d
AC and AD are normals to beams 1 and 2. When beam 1 hits the surface of the
grating at point A, beam 2 still needs to a distance CB to hit the grating surface at B.
For reflected beams 1 and 2 to show constructive interference, the distance CB+BD
should be integer number of wavelengths. Therefore, we can write:
nλ = CB + BD
(1)
where n is an integer called order of radiation.
Angle 1AX + XAC = 900
Angle BAC + XAC = 90o
Therefore, 1AX = BAC = I (incidence angle)
Therefore, CB = AB sin i
(2)
In the same manner:
1'AX + XAD = 90o
BAD + XAD = 90o
Therefore, 1'AX = BAD = r
BD = AB sin r
(3)
AB is the spacing between two consecutive blazes = d, combination between (2) and
(3) and substitution in (1) gives:
nλ = d sin i + d sin r
nλ = d(sin i +sin r)
This relation suggests that there can be several wavelengths for each diffraction angle.
For example:
Wavelength, nm
r, degree
n=1
n=2
n=3
20
748
374
249
10
632
316
211
0
513
256
171
In echelle gratings, usually a large diffraction angle is used where i = r = β
(diffraction angle) and the relation approximates to the following:
nλ = 2d sin β
Filters are called nondispersive λ selectors while prisms and gratings are called
dispersive λ selectors. An instrument which uses a filter as a λ selector is thus referred
to as a nondispersive instrument while an instrument using either a prism or grating as
a λ selector is referred to as a dispersive instrument.
Monochromators
A monochromator is the part of instrument responsible for producing monochromatic
radiation. It is an essential component of any spectroscopic instrument and is
composed of a prism or grating, as the λ selector, in addition to focusing elements;
like mirrors or lenses. All these components are contained in a box that has an
entrance and an exit slit. Two common types of monochromators can be described:
a. Czerney-Turner Grating Monochromator
This is composed of a grating, two concave mirrors and two slits. The following setup
can be associated with this monochromator system:
Focal
Plane
Grating
Entrance Slit
Exit Slit
b. Bunsen Prism Monochromators
This type of monochromators uses a prism as the dispersion element in addition to
two focusing lenses and two slits. The setup can be depicted as in the figure below:
Collimating Lens
Focusing Lens
Focal
Plane
Entranc
e
Exit
Slit
Pris